1. Field of the Invention
This invention relates generally to the art of making bimetallic articles and, more particularly to furnace brazing of two shells with the aim of obtaining a bimetallic cylindrical article. Still more particularly, it relates to a method of brazing of two shells of large diameter viz., a thick-wall outer shell and a thin-wall inner shell.
The invention can find application in the power chemical and petrochemical engineering fields for the production of a large range of vessels, reactors, tubes and pipes.
2. Description of the Prior Art
There is known a method of making bimetallic articles by electric arc deposition (cf., e.g. "Tekhnologia electricheskoi svarki metallov i splavov plavleniem", in Russian, edited by B.E. Paton, 1974, the Masninostroenie Publishers, Moscow, pages 694, 726 to 728), in which a coat layer is applied in several paths by electric arc deposition with wire or tape electrodes and with preliminary and accompanying heating of the article, if base materials used are steels susceptible to cracking during deposition, after which the surface of the layer thus applied is subjected to mechanical machining.
One advantage of this method is obtaining of a reliable bond between the base metal and coat layer, as well as the possibility of applying a coat of a corrosion-resistant layer to cylindrical articles such as cylindrical shells. However, the process of applying a stainless steel coating by electric arc deposition is inefficient and labor-consuming. Other disadvantages are associated with an immediate proximity to the operator of a large mass of heated metal. The layer applied by such a method is non-uniform in thickness and includes flaws in the form of slag, pores and cracks. In addition, the method fails to provide stable corrosion-resistant properties of the deposited layer. There is also known a method of brazing large size cylindrical workpieces (cf., L.G. Puzrin, et al "Avtovakuumnaya vysokotemperaturnaya paika", 1975, the Znanie Publishers, Kiev, pages 8 to 10, in Russian), in which cylindrical pieces are assembled before brazing by arranging one piece in another with a non-capillary clearance over which a braze material is placed. The clearance must necessarily be non-capillary, that is one in which the braze material would be capable to flow under the effect of its own weight, rather than under the action of capillary forces. It is well known (cf., "Spravochnik po paike" edited by I.E. Petrunin, 1984, the Mashinostroenie Publishers, Moscow, page 21), that in a capillary clearance the molten braze material fails to flow to a desired depth, and therefore the clearance must be non-capillary to facilitate penetration of the braze material.
There is a known method (cf., U.S. Pat. No. 3,370,930) bearing the closest resemblance in terms of the technical essence and results obtained to the herein proposed method and used for brazing together a carbon steel outer shell and a stainless steel inner shell, the wall thickness of the outer shell being tens of times the wall thickness of the inner shell. This method is normally used for making large size bimetallic cylindrical articles. The assembly of such an article requires that the thin inner stainless steel shell be preliminarily formed. As the inner shell can be used a section of a pipe or it may be fabricated or by rolling up a stainless steel sheet material, joining its longitudinal edges by a straight weld, and thereafter machining its outer surface. This inner shell is then arranged with a clearance inside the outer carbon steel shell and this clearance being filled with a foil braze material attached to the outer surface of the inner shell. The width of the clearance between the shells is preselected so that between the inner surface of the outer carbon steel shell and the layer braze material there would be a free space of a width equal to the difference in the radial thermal expansion of the shells during subsequent heating, the stainless steel inner shell having a higher coefficient of thermal expansion, than the outer carbon steel shell.
The ends of the shells are joined by vacuum-tight welds through resilient ring elements (compensators) allowing in the course of heating for axial displacement of the ends of the inner shell relative to the outer shell due to the difference in thermal elongation in this axial direction. Air is evacuated from the closed space, that is from the clearance between the carbon steel outer shell and stainless steel inner shell having a layer of braze material applied thereto, the assembly is thereafter heated in a furnace to the brazing temperature, while the gases are being evaluated from the clearance between the inner and outer shells by a vacuum pump. In the course of heating the clearance between the outer shell and the braze material is gradually diminishing, and the shells press each other through the layer of braze material, whereas the provision of resilient ring elements affords free elongation of the inner thin-wall stainless steel shell in the axial direction. Upon attaining the brazing point, the braze material melts to bond the surfaces of the shells, and thereafter the assembly is cooled.
The need for the highly accurate annular clearance between the outer and inner shells is mainly dictated by the preferred assembly procedure during which the finished stainless steel inner shell is arranged inside the carbon steel outer shell, the magnitude of this clearance being strictly limited by the difference between the radial thermal expansion of the shells. In order to obtain the annular clearance of a predetermined uniform width through the whole length of the shells it is necessary to meet certain conditions, each of which may pose a certain technological problem. Such conditions are:
highly accurate finish of the surfaces being bonded together both in terms of shape and dimensions, which is especially difficult with respect to the stainless steel inner shell, which is not rigid enough since its diameter exceeds by far its wall thickness; PA1 evacuating air from the clearance during heating, which is rather inconvenient and technologically difficult, since it requires the provision of a special furnace with conduits connecting the assembly to a vacuum pump and poses extra difficulties associated with placing the assembly into the furnace for brazing; PA1 high accuracy of the assembly procedure which is still more complicated due to the use of the resilient ring elements necessitating a large number of vacuum-tight welds, which makes the assembly procedure still more labor consuming, and also due to maintaining an equal magnitude of clearance between the surfaces to be brazed, a fault of which results in brazing flaws at the side of excessive clearance. All the aforedescribed conditions overcomplicate the prior art method of brazing, which limits the range of its industrial applications.
This method sometimes fails to ensure high quality of the brazed joint due to residual stresses and possible ruptures of the brazed seam caused by the tendency to axial displacement of extreme portions of the stainless steel inner shell relative to the carbon steel outer shell during contraction after cooling, which is especially characteristic for elongated shells.
Furthermore, incomplete mating of the inner shell with the outer shell causes lack of brazing which affect the quality of the brazed structure. When making use of braze material containing such volatile components as manganese and zink, poor brazing may take place due to increased melting temperature and reduced spreading of braze material caused by the entrainment of such components during the evacuation of gases in the course of heating.